The present invention relates to a stretched mask for a color picture tube, which can be used for any type of color picture tube, e.g. a shadow mask tube or an aperture grille tube, in color television and computer color displays.
In color picture tubes for color television and color displays, a mask for color selection is used so that electron beams are applied to predetermined phosphors. As the color selection mask, a shadow mask formed from a metal plate provided with a large number of small holes or an aperture grille provided with a large number of slits is used. When a color picture tube is used continuously for a long period of time, the shadow mask or the aperture grille is heated because accelerated electrons collide against it, and distorted by thermal expansion. This may cause the electron beams to be gradually displaced relative to the phosphor screen, resulting in color shift in the colored image.
In a color selection mask for a color picture tube, a stretched color discrimination mask like an aperture grille, which is stretched on a firm frame, is used together with a member pressed like a general shadow mask.
The stretched color discrimination mask is formed as follows. A hot-rolled low-carbon steel strip containing carbon in units of 0.0001% is cold-rolled to a sheet having a thickness of 0.02 mm to 0.30 mm. After a large number of grid elements have been formed in the cold-rolled steel strip by etching, the steel strip is welded to a frame placed under pressure applied in a direction reverse to the stretching direction. Then, the pressure is removed to form tension by the restoring force of the frame. Thereafter, to prevent the generation of secondary electrons, heat radiation, the formation of rust, etc., the mask material is subjected to heat treatment for 10 to 20 minutes in an oxidizing atmosphere at 450.degree. C. to 470.degree. C., thereby blackening the surface of the mask.
Conventionally, there is a likelihood that the tension of the grid elements of the color discrimination mask may reduce during the production, and this is a matter of great concern in quality control. The problem is due to the fact that the grid elements elongate by creep caused by heat and tension during the blackening of the color discrimination mask material. The grid elements having a low tension recovery factor and lowered tension because of large creep have the problem that if vibrations are applied to the grid elements, for example, when the sound level of a speaker provided in the same cabinet as that for the color picture tube, the grid elements themselves vibrate with large amplitudes, causing color shift in the colored image.
To solve the problem, JP2548133 (B2) discloses a color selection mechanism formed from a low-carbon steel sheet containing 40 ppm to 100 ppm of nitrogen. JP2683674 (B2) proposes a low-carbon steel sheet containing 0.20 to 5 2.0% by weight of Cr and 0.10 to 3.0% by weight of Mo. However, these low-carbon steel sheets have the problem that because of large residual stresses, the tape portion of the aperture grille is unfavorably twisted after the heat treatment.
JP799025 (A) (U.S. Pat. No. 5,552,662) discloses a method of producing an aperture grille using a material having small residual stresses. However, because the tensile strength is low, there is almost no change in the tension recovery factor. Therefore, the tape of the aperture grille may break when the aperture grille is stretched. If the aperture grille is stretched under a tension with which the tape will not break, the stretching tension reduces undesirably after the heat treatment.
An object of the present invention is to provide a stretched color selection device for a color picture tube that has minimal residual stresses and is free from problems such as twisting and that has a high tension recovery factor.